The present invention relates to a method for allocating subscriber addresses to bus subscribers in a bus-based control system for the automated control of a technical process. The invention also relates to a bus-based control system for the automated control of a technical process that uses such a method.
A bus system is a communication network which is set up specifically to allow data communication between a plurality of what are known as bus subscribers and a central station, if appropriate. Typically, bus systems are used in order to allow cyclic data communication between controllers and I/O devices which are physically remote from one another. The I/O devices have inputs and/or outputs which are used to receive signals from remote sensors and to actuate remote actuators, respectively. Controllers process input data corresponding to the signals from the sensors and produce output data from which the I/O devices produce the output signals for the actuators. The use of bus systems allows a plurality of sensors and actuators to be arranged physically away from one or more controllers with limited cabling complexity.
There are numerous variants of bus systems which have been developed for different requirements. In many cases, the bus subscribers each require an individual subscriber address so that the messages transmitted by means of the communication medium can be associated with the desired receiver, and to some extent also in order to identify the respective sender of the message. The subscriber addresses are usually allocated in a configuration mode before the bus system is put into operation.
Various methods for allocating subscriber addresses are known. One method involves providing the individual bus subscribers with address selector switches, wherein the respective subscriber address is set on the bus subscriber itself by using the respective address selector switch. Such an approach has the disadvantage that a subscriber address can be allocated only at the location of the respective bus subscriber. Central configuration of the bus system is not possible.
There are therefore numerous methods which can be used to assign addresses to bus subscribers in a bus system from a central station via the bus system itself. One such method is described in DE 197 33 906 A1, for example. In this case, the address allocation is based on different distances between the bus subscribers and what is known as the master station, which allocates the subscriber addresses, and also on the resultant different message transfer times. The method is used to assign a respective individual subscriber address to a plurality of bus subscribers one after the other. However, the known method is not suitable for the allocation of subscriber addresses in a bus system which is intended to be used for transmitting safety-related data in terms of European standards EN 954-1, IEC/EN 61508 and EN ISO 13849-1. These standards define requirements for the design and implementation of machines and machine installations which may present a hazard to persons during operation. If, by way of example, the bus system is intended to be used to transmit an enable signal from an emergency off pushbutton or a safety light barrier to a controller or a shutdown signal for an electric drive, it is necessary to ensure that the respective messages come from the correct sender and also reach the correct receiver. An erroneous address assignment can have fatal consequences, for example if an incorrect machine is shut down when a safety light barrier is broken, and therefore a hazardous machine continues to run.
For a bus system which is intended to be used to control safety-related events within the terms of the aforementioned standards, the address allocation needs to be failsafe. In this context, “failsafe” means that at least the requirements of category 3 of European standard EN 954-1 and/or the requirements according to SIL2 of IEC/EN 61508 or comparable requirements are met. A possible error in the allocation of the addresses must accordingly not result in the controlled installation entering a state which presents a hazard to a user or another person.
DE 199 34 514 C1 discloses a method for allocating subscriber addresses to bus subscribers in a control system, in which the individual bus subscribers have an explicitly defined physical position as a result of the structure of the bus system. Each bus subscriber uses its explicit physical position to verify an assigned logical address. The method is particularly suitable for the allocation of subscriber addresses to bus subscribers on what is known as the Interbus, which has widely been used for the automation of technical processes in production environments. However, verifying an allocated subscriber address by using an actual physical position uses a particular property of the Interbus and therefore cannot be transferred to other bus systems without this particular property.
DE 199 40 874 A1 discloses another method for configuring a bus subscriber for a safety-oriented control system, wherein the bus subscriber is assigned a subscriber address. In this case, an unconfigured bus subscriber uses a predefined, explicitly stipulated universal address to register with a central station which is likewise connected to the bus system. The central station assigns the unconfigured bus subscriber a subscriber address which is verified by virtue of subsequent data traffic between the bus subscriber and the station. In this case, the central station monitors that only precisely one bus subscriber is configured in each case. This method is particularly suitable for configuring an individual bus subscriber which is used as a replacement device to replace a faulty bus subscriber on an already configured bus system. If a relatively large number of bus subscribers are intended to be assigned addresses, this method is very time-consuming, however.